I thought I would give my input here as I came across the thread on a related issue, which I have now fixed thanks to suggestions above.
Pre 1.2 I was playing on 1920x1080 with 1920x1200 as my native resolution.
Upon relaunching the client it then refused to letterbox the game and changed my native resolution to whatever the client was set to.
Following some of the advice above (as I have NVIDIA) I adjusted 2 settings under the "adjust desktop size and position". I put my scaling mode to "no scaling" and checked the "override the scaling mode set by games and programs".
I can now run the game back in letterbox, happy days.
Hope this helps someone.
Nope, dont work for me
When I do that i got some black bar on bottom of screen, dont know why